The 46-year-old was frustrated to see Mats Hummels' second-half strike disallowed and admits the DFB-Pokal final loss to Bayern Munich was "very painful"
Extra-time goals from Arjen Robben and Thomas Muller sealed back-to-back domestic doubles for the Bavarians, although Dortmund felt aggrieved that Mats' Hummels second-half strike - which replays showed crossed the line - was not spotted by the referee.
Klopp revealed the entire Dortmund dug-out were left stunned that the goal was not awarded but stressed he was proud of his side's season on the whole.
"We scored a goal that didn't count. All of my substitutes saw that," Klopp said to Sky after the match.
"I don't want to pin the blame on the referee, but the goal would have changed a lot. We lost 2-0 even though we scored a goal. The referee Florian Meyer told me you need the goal-line technology for those things.
"The loss is very painful. Both teams pushed themselves to the limit. We lacked concentration for the goals but any time tonight I will tell the boys what an extraordinary season we produced."